An in-vehicle ECU may receive output signals from various sensors, which are input into the in-vehicle ECU, as disclosed in a patent document 1 (i.e., Japanese Patent Laid-Open No. 2005-204080). When a signal is input into the in-vehicle ECU, the signal is processed by electronic circuits within the in-vehicle ECU. After the processing, the original signal is divided into two signals. One of the divided signals (i.e., a first signal) may be directly input into a first input terminal of a multi-channel A/D converter for an A/D conversion without a change in amplification. The other signal (i.e., a second signal) may be input into a second input terminal of the A/D converter for an A/D conversion after the signal is amplified by an amplification circuit.
When the sensor output signal that is input into the in-vehicle ECU has a low voltage level, the in-vehicle ECU uses an A/D conversion value of such a signal that is based on the second signal (i.e., a second A/D conversion value). When the sensor output signal that is input into the in-vehicle ECU has a high voltage level, the in-vehicle ECU uses an A/D conversion value of such signal based on the first signal (i.e., a first A/D conversion value).
During the processing of the second signal, an error may be introduced into the second signal. The error may be introduced into the amplification circuit by a change of an environmental factor or a usage environment, such as, for example, temperature variations of the ECU or the age of the circuit 16. Therefore, according to the patent document 1, such an error is corrected by (i) assuming that the input-output relationship of the amplification circuit is a linear function, and (ii) inputting two signals to the in-vehicle ECU respectively as the sensor output signals, which are (a) a signal having a constant 0 voltage (i.e., a 0V signal) and (b) another signal having a constant specified voltage (i.e., a specified voltage signal). More practically, to correct the error in the signal, the linear function that defines the input-output relationship of the amplification circuit is estimated based on the first A/D conversion value and the second A/D conversion value, which are respectively derived from the 0V signal and the specific voltage signal that is input into the in-vehicle ECU. The error correction is performed by using the estimated linear function.
However, when estimating the linear function, both the first A/D conversion value and the second A/D conversion value, which are respectively derived from the 0V signal and the specific voltage signal, may include errors caused by noise and the usage environment. When estimating the linear function based on such error-ridden conversion values of the first and second signals, the accuracy of the linear function estimation may improve with a greater a voltage difference between the first and second signals.
However, according to the patent document 1, the A/D converter has an inputtable voltage range with a specific upper/lower limit value of 5V and 0V, for example.
In this case, since the correction method in the patent document 1 uses first and second signals respectively having constants (i.e., 0V signal and the specific voltage signal), it is impossible for the correction method in the patent document 1 to increase the voltage difference between the first and second signals in order to accurately estimate the linear function that represents the amplification circuit. As a result, the correction method in the patent document 1 cannot accurately estimate the linear function and correct errors in the sensor output signal caused by signal processing of the amplification circuit.